Lactation is critical to survival of all mammals because it supports the growth of the newborn. Successful lactation requires completion of all developmental stages proceeding lactation, including pubertal development largely stimulated by estrogen, expansion of the secretory alveoli in early pregnancy driven by progesterone and prolactin, secretory differentiation occurring during late pregnancy, and secretory activation that occurs at parturition. A failure at any stage often results in ineffective lactation, indicated by either inadequate growth or death of newborns. Despite substantive advances in mammary gland biology, substantial gaps in our understanding of lactation exist. While great advances have been made regarding regulation of milk protein genes, our understanding of lipid biosynthesis has lagged behind. During the last ten years our group at the University of Colorado School of Medicine, has focused on secretory differentiation and activation with an increasing focus upon lipid metabolism and synthesis of lipid component of milk. The overarching hypothesis of this program project grant is that mammary gland development is not controlled by a single master controller gene, but rather a carefully patterned progression of genetic regulators that control development during mid-pregnancy, late pregnancy, and secretory activation. In Project I we will examine the role of Aktl in regulating the lipid switch that occurs at secretory activation- the coordinate transcriptional activation of enzymes involved in lipid biosynthesis. In Project II we will examine the role of progesterone and insulin in alveolar development during pregnancy. In Project III we will examine the role of microRNAs in mammary gland development with particular focus on their ability to modulate metabolic changes and signaling by steroid hormone receptors. In Project IV we will focus on how formation of cytoplasmic lipids droplets is regulated. These projects are assisted by two Cores: an Administrative Core that oversees fiscal issues and provides logistic support and the Adenovirus Core that prepares recombinant human adenoviral vectors for use in the projects. This concerted effort will provide mechanistic underpinnings for understanding lactation failure in women related to diabetes and/or obesity.